1. Technical Field
The present invention relates in general to computer buses, and in particular to a method and apparatus for concurrent connection transition of a peripheral device to a computer bus to establish both power and 2-way data transfer without interrupting power to other devices on the computer bus or causing data corruption on the computer bus.
2. Description of the Related Art
Various methods and apparatus for the inserting or removing peripheral devices or control circuits to computer buses are known in the art. In an effort to minimize the impact of plugging into a bus, the normal procedure has been to shut down the bus so that new devices would not disrupt data flow on the bus. An alternative has been the hot-plugging concept where both power and data transfer interconnection are achieved by directly plugging the device into power and bus connectors that are both powered and active. Hot-plugging is found in fault tolerant systems which normally include device or field replaceable unit redundancy couple operational comparison and checking logic to ensure correct operation. When a fault is detected, an indication of the failing device is provided to service personnel. The failing device is then simply removed from the bus and a replacement device connected. The removal of the failing device and the replacement of a new device are performed without regard to ongoing bus activity. Both the bus architecture and the control device electronics must be carefully designed to achieve this xe2x80x9chot pluggingxe2x80x9d capability.
The normal control circuits contain electronic microchips mounted on printed circuit (p.c.) boards, and circuit interconnections to the bus and for power and data transfer are made via edge connections. Edge connectors are mounted on a p.c. board which is plugged into a corresponding receptacle for connection to the bus. Plugging in the board makes electrical contact between the edge connectors and the corresponding bus receptacle and thus provides both power to the electronic components on the p.c. board and interconnects it with the bus in one operation. For hot plugging, the normal method of interconnection is to increase the length of at least the ground contact on the edge connector, so that a ground contact can be completed prior to the electrical connection of the other contacts for the application of power and the transfer of data signals. Without strict control of data design parameters of the bus architecture and control circuits, hot plugging such circuits into a computer bus likely will cause disruption in the two-way transfer of information on the bus.
This data disruption on the bus is often seen in networks using the standardized Small Computer System Interface (SCSI) bus. This standard defines the mechanical, electrical, and functional requirements for a small computer input/output bus and command protocols to enable attaching small computers with each other and with intelligent peripheral devices such as rigid or flexible disks, magnetic tape drives, printers, optical disks and other direct access storage devices (DASD), etc. Such devices will hereafter be referred to as either peripheral devices or more generally as electrical devices. The primary objective of the standard is to provide host computers with device independence within a class of electrical devices to enable a variety of different devices to be added to host computers without requiring modifications to generic system hardware or software.
When performing concurrent maintenance on peripheral devices connected to the SCSI bus while the bus remains in operation, the peripheral device may be inserted or removed from the SCSI bus while the system is running and the other devices are communicating over the bus. Such xe2x80x9chot-pluggingxe2x80x9d of an electrical device on the SCSI bus disrupts on-going transfer on the data bus by introducing electrical spikes and glitches that may cause data corruption. Further, power spikes and glitches on the power connector may cause failure of the power supply or an overcurrent fault. In order to avoid this possibility of data corruption or power supply interruption, some prior art systems turn power off to the device that is being hot-plugged. In some systems, which device is to be serviced is communicated to a power control network by a SCSI bus initiator over a special power control bus. The SCSI bus initiator then indicates to the power control object when to remove power and to restore power to the device being serviced. In other systems, special hot-plugging logic is included in the power control system of the device slots to allow a device to be pulled and inserted into the bus system while power remains on the rest of the electrical devices. What is clearly needed is a power control system for a SCSI bus that does not require an additional hardware path to support power communications or complicated and expensive hot-plugging circuitry, while still maintaining data integrity of the bus and power integrity of the power supply when electrical devices are removed and inserted on an active SCSI bus.
A method and apparatus that prevents data corruption and power supply failure caused by device connection transition on an active SCSI bus is provided. A bus initiator pauses communication on the SCSI bus such that only communication by the bus initiator is allowed over the data communication bus. All other communication between peripheral devices attached to the data communication bus is paused. A shutdown signal from the bus initiator is transmitted over the data communication bus to a power controller, which eliminates power to a device slot corresponding to the received shutdown signal. After a connection transition of an electrical device in the device slot, a power reset signal is transmitted from the bus initiator over the data communication bus to the power controller, which then restores power to the device slot upon receiving the reset power signal. The bus initiator then releases the data communication bus from being paused such that all communication on the data communication bus by peripheral devices is allowed. The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed written description.